Saccadic eye-movements reduce but do not eliminate the line-motion illusion

Burr et al (1994) have proposed a selective suppression of the M- pathway during voluntary saccades. An alternative explanation is that transient attention is selectively suppressed. To test this idea, the strength of line-motion attention (Hikosaka et al 1993) was measured within and between saccadic eye movements. The strength of the illusion was measured by a nulling technique under 4 conditions: cue presented during or after a voluntary saccade and no-cue trials with similar during/after saccade presentation. Saccadic eye movements were detected using a Skalar infra-red eye movement detector which triggered cue presentation immediately (within 8 msec) or with an extra 85 msec delay (i.e. after the saccade was complete). Logistic fitting of the response of 6 subjects (each completing over 350 trials) showed that with achromatic stimuli, the cue effect (as measured by the shift in the psychometric function) was reduced from 34 msec to 17 msec relative to trials with no cue. A non-zero line-motion illusion when the cue is presented during a saccade suggests the cue-line facilitation occurs in in body-space rather than in retinotopic coordinates. Together with visual evoked potential (VEP) data showing that saccades do not systematically alter the M-pathway early cortical response, the reduced cue effect on the line-motion illusion under the saccadic state could be interpreted as hypothesised, namely, as being due to a reduction in the transient neural facilitation (attention) that would normally occur in the neighbourhood of the cue